utmp: rewrite utmp logging

This patch generalizes the utmp support, to not only support
libutempter, but also ulog (and in the future, even more interfaces).

* Rename config option main.utempter to main.utmp-helper
* Add meson option -Dutmp-backend=none|libutempter|ulog|auto
* Rename meson option -Ddefault-utempter-path to -Dutmp-default-helper-path
* utmp is no longer detected at compile time, but at runtime instead.

Meson will configure the following pre-processor macros, based on the
selected utmp backend:

* UTMP_ADD - argument to pass to utmp helper when adding a record (starting foot)
* UTMP_DEL - argument to pass to utmp helper when removing a record (exiting foot)
* UTMP_DEL_HAVE_ARGUMENT - if defined, UTMP_DEL expects an extra argument ($WAYLAND_DISPLAY)
* UTMP_DEFAULT_HELPER_PATH - path to the default utmp helper binary

The documentation has been updated to mention which arguments are
passed to the helper binary.

Closes #1314

doc/meson.build

@@ -2,16 +2,26 @@ sh = find_program('sh', native: true)
 
 scdoc_prog = find_program(scdoc.get_variable('scdoc'), native: true)
 
-if utempter_path == ''
-  default_utempter_value = 'not set'
+if utmp_backend != 'none'
+  utmp_add_args = '@0@ $WAYLAND_DISPLAY'.format(utmp_add)
+  utmp_del_args = (utmp_del_have_argument
+                   ? '@0@ $WAYLAND_DISPLAY'.format(utmp_del)
+                   : '@0@'.format(utmp_del))
+  utmp_path = utmp_default_helper_path
 else
-  default_utempter_value = utempter_path
+  utmp_add_args = '<no utmp support in foot>'
+  utmp_del_args = '<no utmp support in foot>'
+  utmp_path = 'none'
 endif
 
+
 conf_data = configuration_data(
   {
     'default_terminfo': get_option('default-terminfo'),
-    'utempter': default_utempter_value,
+    'utmp_backend': utmp_backend,
+    'utmp_add_args': utmp_add_args,
+    'utmp_del_args': utmp_del_args,
+    'utmp_helper_path': utmp_path,
   }
 )